A downstream headwall and wing-walls at Lake Mare Prairie. Photos: Orange County Public Works Department

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An upstream area of dislodged rip-rap and constant seepage at Lake Conway.

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The sheet pile was installed along the western edge of the weir footing May 19 – 21, 2009.

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The sheet pile was structurally connected using a reinforced coping beam May 21, 2009.

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Grouting was pumped into each point during June 5 – 8, 2009.

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Daetwyler weir structure repair

DESIGN:$52,960

CONSTRUCTION:IN-HOUSE CONSTRUCTION CREW:$144,251

IN-HOUSE HEAVY EQUIPMENT CREW: $97,852

IN-HOUSE DRAINAGE CREW: $35,401

GROUTING BY ENVIROWASTE: $14,873

TOTAL: $292,376

In late 1999 a culvert near the Orlando International Airport in Orange County, Fla., collapsed — likely the result of buried muck and peat on the northwestern side of the structure where seepage had appeared. The structure is the outfall control for Lake Conway, located on Daetwyler Drive about half a mile north of McCoy Road near the airport. It was replaced by a Conspan culvert, Keystone retaining walls, concrete weir on gravel foundation, and re-no mattresses for erosion protection.

Seepage rates under and around the weir/wing-wall/culvert structure are influenced by the head differences or differential in water elevations across the structure as well as the direction of flow.

The western side of the structure (along Lake Conway) is considered the upstream location since it generally has higher water level, and the eastern side is a canal connecting to Lake Mare Prairie. The water levels in Lake Mare Prairie fluctuate, and during the rainy season they are higher than in Lake Conway where flow reversal occurs. Manifestations of high seepage seem to appear when the head difference is greater than 1.5 feet but becomes more pronounced when it approaches 3 feet.

Lake Conway and Lake Mare Prairie have different hydrologic regimes due to the nature and area of the contributing watersheds and their bottom leakage characteristics. Lake Conway is prone to sinkholes, whereas Lake Mare Prairie will likely have less vertical water loss through its lake bottom. The implication of this hydrogeology difference is that Lake Mare Prairie will not decline as rapidly as Lake Conway during droughts.

Lakefront residents in the area had expressed serious concerns over differential flow problems, and the potential for soil loss and erosion, under the existing Conspan culvert and weir structure at Daetwyler Drive.

Devo Engineering conducted a geo-technical investigation in April 2009 to investigate the seepage and related soil loss under the weir, Conspan footing, and behind the wing-walls. The investigation found that:

The weir concrete footing was constructed on a foundation of crushed rock, which — even though well compacted — would have some degree of permeability, raising the possibility of more seepage.

During periods when there was a large head differential across the weir, the seepage forces would have been great enough to cause the loss of fine grained soils from beneath the weir footing.

Historical changes in lake levels that resulted in flow reversal on several occasions had caused additional soil loss, which further exacerbated the situation by creating some voids and structural distress.

Probes disclosed some voids in the base material, and some settlement of the base material appeared to have occurred. This settlement is consistent with a loss of fine soils from the base material.

There was no evidence of swirling water, eddies, vortices, or air bubbles within the “downstream” pool that indicated the presence of a principal cavity and associated flow path.

A tracer-dye test disclosed significant seepage taking place under the weir footing, which is more prevalent in the northern and northwestern portions of the weir.

A higher level of assurance and medium-term performance results when the first recommendation is combined with the cutoff wall along the western edge of the existing weir. However, this option would add problems of working in close proximity to overhead power lines and access constraints.

Construction of a new weir east of Daetwyler Drive would provide the highest level of assurance and long-term performance and lends itself to ease of construction and very little inconvenience to road users. However, it would need a permit from the water management district.

A combination of the first two recommendations was used to retrofit the weir. Work performed included:

Pressure-grouting of the voids within the granular stone layer beneath the weir foundation. This includes coring at strategic locations of the concrete base slab to facilitate the installation of grout injection pipes at intervals not exceeding 5 feet. This approach ensures that the cement grout reachs the intended target areas without requiring pumping pressures high enough to cause uplift and deformation of the weir structure. The injection process had to be properly balanced by applying grout through alternating pipes at controlled pressures not exceeding 50 psi.

Installation of a sheet pile cutoff wall along the western edge of the weir footing, extending at least 5 feet into natural soils beyond the high water mark in the canal. The sheet pile was structurally connected to the weir structure using a reinforced concrete coping beam with dowel bar connectors.

The retrofit activities were very successful but several challenges were encountered:

Presence of overhead power lines could have hampered the installation of sheet piles; safety practices in the swing movements of the heavy equipment were used.

Access and working space at the sides of the weir structures were limited.

Rising lake levels due to heavy rainstorms in the month of May and June were hazardous. Dewatering was necessary in order to pour concrete into the coping beam where the sheet pile and injection pipes are located. Dewatering was done with sandbags in the Lake Conway side, and a dirt dike on the Lake Mare Prairie side together with water pumps helped prevent rising water from the construction area.

Workers poured concrete on the coping beam to reinforce the pile placement. Despite the challenges, staff and contractors safely retrofitted the weir to provide flood protection and ease public concerns with rising waters from Lake Conway and Lake Marie Prairie.

— Deodat Budhu is the manager and Elizabeth White is an engineer with the Orange County Public Works Department's Roads and Drainage Division.